In a commercial or institutional kitchen there is a conflict between the need to maintain a comfortable environment for the kitchen personnel while withdrawing from the kitchen environment sufficient air to accommodate an exhaust hood which is designed to withdraw the plume rising from cooking or heating devices such as deep fat fryers, ovens, ranges, flame cookers, grills, and the like. These plumes may include heated air, particulates of water vapor and/or organic material released from the food being cooked or heated and entrained in the heated air that rises from the heating or cooking device, and/or fumes from combustion heating elements. The velocity of these plumes depends largely upon the surface temperature of the heating or cooking device and tends to vary from about 15 fpm over steam equipment to about 150 fpm over charcoal broilers, for example. These plumes, if undisturbed by extraneous air currents, will normally rise from the cooking or heating surface. Under these undisturbed conditions, the plume is readily captured by a canopy type exhaust hood and conveyed to a discharge location outside the kitchen. When the plume is subjected to extraneous air currents, such as those generated by HVAC equipment serving the kitchen, movement of personnel past the hood, etc., the flow of the plume is disturbed and tends to not be readily captured by the exhaust hood.
In the prior art, the actual quantity of air and flow velocity of the air to be used to capture and exhaust a plume is determined by the flow rate of the plume, plus a safety allowance to absorb cross-drafts, flare ups, etc., plus a safety factor for the style of hood. Gas-fired equipment may require an additional allowance for the intake of combustion air and exhaust of combustion products. In the prior art, it is known that the use of a single hood over a single heating or cooking device is impractical so that groups of devices should be disposed beneath a single hood, and that optimum efficiency of operation, hence cost, of a hood is achieved when the all the heating and cooking devices disposed beneath a single hood have the same profile, i.e. the devices should all have their heating or cooking surfaces at or near the same horizontal level. Tall ovens, for example, should not be disposed under the same hood as ranges, charbroiler, etc. wherever possible. First, the taller oven requires that the hood be located at a relatively high horizontal level which placed the hood inordinately far above the source of the plume from a charbroiler, for example. This, in turn, requires that the exhaust air flow rate for the hood be greater so as to ensure capture of the plume from the charbroiler (a low profile device). Obviously, the economics of this situation are not desirable.
In the prior art, the exhaust flow rate (cfm) of a hood is based upon, among other things, the type of hood, e.g. wall mounted canopy, backshelf, island canopy, etc., the equipment under the hood, the flow rate required for the hood being based on the flow rate required to exhaust the plume from that heating or cooking device which generates the greatest heat, and presumably the greatest plume. In any event, there must be sufficient volume of air entering the hood which will provide the required exhaust flow rate for the hood. Heretofore these requirements have been sought to be met by one of two methods. In a first method, a somewhat arbitrary exhaust air flow rated, in cfm, is chosen, based largely upon the type of equipment to be disposed beneath the hood, and this air flow rate is multiplied by the area, in ft.sup.2, of the entrance opening to the hood. Using this method, a hood having an entrance opening of 26 ft.sup.2 and which is to service a charbroiler and other low profile devices, the arbitrary number commonly is at least 200, thereby requiring an exhaust air flow rate for the hood of 5200 cfm. In a second method, a substantially larger, but still somewhat arbitrary air flow rate, in cfm, is chosen, again based largely upon the type of equipment to be disposed beneath the hood, and this value is multiplied by the linear feet of hood. For a 13 ft. long hood, servicing a charbroiler and other low profile devices, an arbitrary number of 300 is commonly chosen for this calculation. In this example, the hood should have an exhaust air flow rate of 3900 cfm.
Conventional teaching in kitchen ventilation holds that the exhaust flow rate should at least equal or exceed the plume flow rate. This ensures both that the total of the plume volume will be exhausted through the hood and that the velocity of the plume flow is sufficient to capture and retain therein the particulates associated with the plume. The only material consideration given in the prior art to such factors as the great diversity of type of heating or cooking devices that may be disposed beneath a given exhaust hood, the great diversity of plumes which may be generated by the devices, and the large number of environmental factors that may exist outside the hood, but within the kitchen environment, has been the inclusion of a safety factor in the calculation of the minimum exhaust hood air flow rate. As a consequence, the prior art hoods tend to be grossly oversized, hence costly to fabricate, install and operate. Further, in the prior art, as circumstances change following the initial design and installation of a hood, such as substitution of a different cooking device under the hood or other changes which affect the required capture velocity, the prior hoods have not possessed the capability to change the capture velocity of the air drawn into the hood, aside from changing the exhaust fan capacity of the hood. This latter action serves to exacerbate the over sizing of the hood.
Obviously, air exhausted by a hood of the non-makeup air type must come from the kitchen environment. The environment of the kitchen most commonly is "tempered", i.e. conditioned or heated, depending upon the season of the year, etc. Providing this tempered air is costly so that desirably only a minimum volume of this tempered air is exhausted by the operation of the exhaust hood.
Still further, its is conventional prior art teaching that canopy hoods should overhang the cooking or heating equipment serviced by the hood, often as much as 12 inches overhang in the front of the hood. This overhang requirement requires that the most lower front edge of the hood be at least about six and one-half feet above the floor to avoid kitchen personnel bumping into the hood with their head. This requirement places a further requirement on these prior art canopy hoods in that the distance between the cooking or heating surface of the equipment and the entrance to the hood becomes excessive and added exhaust flow capacity is required to prevent the plume from expanding and escaping from under the hood before it can be drawn into the hood.
It is an object of the present invention to provide an exhaust hood of the non-makeup air type for exhausting the plumes from a plurality of diverse heating or cooking devices disposed beneath the hood, and which minimizes the total volume of tempered air removed from the kitchen atmosphere ambient to and outside the hood.
It is another object of the present invention to provide an exhaust hood which cost-effectively ensures proper exhaust volume of the hood when servicing a plurality of diverse heating or cooking devices disposed there beneath.
It is another object of the present invention to provide an exhaust hood which has no forward overhang and may be set-back from the most forward edge of the heating or cooking equipment serviced by the hood.
It is another object of the present invention to provide an exhaust hood which is amenable to ready and easy physical alteration thereof in a manner that optimizes the capture velocity of air moving from the kitchen environment, over the heating or cooking devices, into a plenum defined within the hood, and thereafter exhausted externally of the hood and kitchen.